1. Field of the Invention
The present invention relates to the field of piezoelectric resonators whose primary application is for signal filtering and reference oscillators.
2. Prior Art
The present invention pertains to piezoelectric resonators whose primary application is for signal filtering and reference oscillators. These resonators are commonly referred to as BAW (bulk acoustic wave resonators). Other acronyms for the same or similar devices include FBAR (film bulk acoustic resonators) or SMR (solidly mounted resonators), or TFR (thin film resonators) or SCF (stacked crystal filters).
The resonators must be as efficient as possible in terms of energy losses. These devices are not new and are well documented in the literature.
Standard IC fabrication methods are used for the basic manufacturing sequences, including depositions, photolithography, and etch processes. MEMS techniques may also be employed for packaging and resonator acoustic isolation from the substrate.
A Bragg mirror is used for acoustic isolation in SMR devices. In FBAR devices, the resonators are built upon a membrane. Both types of isolation are designed to prevent energy loss in the device.
Key aspects of a bulk acoustic wave resonator (BAW) are the quality factors (Q). The Q values are dominated by electrical and acoustical losses. Connections to the resonator need to minimize electrical resistance and also avoid interference with the acoustic balance of the device.
One method of contacting the top electrode is to use the top electrode metal itself for routing to a position well outside of the device, as shown in FIG. 1. This is the common practice. It has several disadvantages:
1. The top electrode sheet resistance is typically about 10 times higher than an average low resistance metal interconnect layer.
2. The top electrode defines the device area. By allowing the active device area as defined by the top electrode to extend beyond the bottom electrode edge there are potential additional parasitic acoustic modes excited.
3. At the bottom electrode edge there are usually defects and poor crystal orientation of the piezoelectric material that can interfere acoustically. It is preferable to not cross these boundaries with the top electrode active area.
If the resonator architecture includes non-planarized topography (e.g. from the patterned Bragg reflector layers) then the deposition and pattering of the thin top electrode layer over the topography can result in higher resistance.